Chain and sprocket systems are often used in automotive engine systems to transmit rotational forces between shafts. For example, a sprocket on a driven shaft may be connected via a chain to a sprocket on an idler shaft. In such a chain and sprocket system, rotation of the driven shaft and driven sprocket will cause the rotation of the idler shaft and idler sprocket via the chain. In an automotive engine system, sprockets on the crankshaft may be used, for example, to drive one or more cam shaft sprockets.
The chains used in chain and sprocket systems typically comprise a plurality of intermeshing link plates connected with pins or rollers. The sprockets typically comprise a circular plate having a plurality of teeth disposed around the circumference thereof. Located between adjacent teeth are roots having generally arcuate or semi-circular profiles for receiving the pins or rollers of the chain. Each root has a root radius, defined as the distance from the center of the sprocket to a point on the root closest to the sprocket center.
In a “straight” sprocket the root radii are all substantially equal. However, it has been found that as a chain rotates around a straight sprocket, audible sound frequencies creating undesirable noise are often generated as the pins or rollers connecting the links of the chain contact the sprocket teeth and impact the roots disposed between adjacent teeth of the sprockets.
The sound frequencies and volume of such noise typically varies depending on the chain and sprocket designs, chain rotational speed, and other sound or noise sources in the operating environment. In the design of chain and sprocket systems, it can be desirable to reduce the noise levels generated as the rollers of a chain engage roots of a sprocket.
“Random” sprockets have been developed to help reduce the radiated noise levels generated by the engagement of the chains with the sprockets. Random sprockets may be characterized by having a plurality of different root radii. The different root radii can be arranged in a pattern around the sprocket circumference to modulate the sound frequencies generated by the engagement of the chain rollers or pins with sprocket teeth and roots. By modulating these sound frequencies, the noise generated as the chain rotates around the sprocket may be reduced.
In addition to minimizing noise generated by engagement between a chain and sprocket, it is also desirable to reduce the tensions imparted to the chain by the sprocket. Reduced chain tensions can be advantageous because they may result in decreased wear of the chain, thus increasing the life cycle of the chain. Furthermore, reduced chain tensions may also result in less wear to the sprocket, thereby also increasing the life cycle of the sprocket.
It has been observed in chain tension measurements that certain chain tensions in a particular system may vary on a periodic or repeating basis, which often can be correlated to tension inducing events. For example, in automotive timing chain systems, it has been observed from chain tension measurements that the engagement and disengagement of each sprocket tooth and/or root with the chain pins often results in repeating tension changes. These chain tension changes may be correlated with potentially tension inducing events, such as the firing of piston cylinders, transmission engagements, etc.
A useful approach to analyzing such tension events is to observe the number of events that occur relative to a reference time period, as well as the amount of the tension change for each event. For example, in an automotive timing chain system, one may observe the number or frequency of tension changes in the chain relative to rotations of a sprocket or a crankshaft, as well as the magnitude of the tension change in the chain.
In such a system, for example, a tensioning event that occurs once per shaft or sprocket rotation may be considered a “first” order event, and an event occurring four times for each shaft or sprocket rotation may be considered a “fourth” order event. Depending on the system and the relative reference period, i.e., rotations of the crankshaft or the sprocket (or another reference), there may be multiple “orders” of events in a crankshaft or sprocket rotation in such a system originating from one or more tension sources. Similarly, a particular order of the sprocket rotation may include or reflect the cumulative effect of more than one tensioning event. As used herein, such orders of tensioning events occurring during a sprocket (or crankshaft) rotation also may be referred to as the orders of a sprocket (or crankshaft) and/or sprocket orders (or crankshaft orders).
In straight sprockets, measurable tensions typically are imparted to the chain at a sprocket order corresponding to the number of teeth on the sprocket, also known as the pitch order. Thus, in a sprocket with nineteen teeth, tensions would be imparted to the chain at the nineteenth or pitch order, i.e., nineteen times per revolution of the sprocket. Depending on the sprocket design, the order in a straight sprocket would typically occur at equal intervals relative to the sprocket rotation, with a generally equal tension change or amplitude.
Random sprockets, in contrast, typically have different tensioning characteristics when compared to straight sprockets due to the different root radii. As the chain rotates around the random sprocket, each of the different root radii typically imparts a different tensioning event to the chain. For instance, as a roller of the chain engages a root having a first root radius, the chain may be imparted with a tension different from when a roller of the chain engages a root having a second root radius larger than the first root radius. Tension changes, in addition, may also be imparted to the chain by a random sprocket due to the relative positioning of the different root radii. A roller moving between adjacent roots having the same root radii may result in different chain tension changes than a roller moving between adjacent roots having different radii.
The change in chain tensions imparted by random sprockets may be further accentuated when the sprocket has more than two different root radii. In a random sprocket having first, second, and third successively larger root radii, the tension imparted to the chain may be greater when a chain roller moves from a root having a first root radii to a root having a third root radii than when a chain roller moves from a root having a first root radii to a root having a second root radii.
Thus, random sprockets designed principally for noise reduction often cause increases in chain tensions and tension changes as compared to the maximum tensions imparted to the chain by straight sprockets. For example, a random sprocket design may reduce chain noise or chain whine by reducing the pitch order of the sprocket. However, reducing the pitch order of a sprocket may result in redistributing or concentrating the tensional forces imparted to the chain by the sprocket over the lower orders of the sprocket. This often results in increased chain tensions corresponding to the lower orders of the random sprocket.
The increased chain tensions at the lower sprocket orders frequently cause the overall maximum chain tension force exerted on the chain and sprocket to increase. As a consequence, a chain and sprocket system subjected to such tensions typically will experience greater wear and increased opportunities for failure, as well as others adverse effects, due to the concentration of the tensional forces in the lower orders.
Accordingly, there remains a need for a sprocket design and method of designing sprockets that incorporates the noise reduction properties of random sprockets without the increased maximum chain tensions associated therewith. In addition, there is a need for a sprocket design and method for designing sprockets to provide the flexibility to shift tensional forces to different sprocket (or other) orders to enhance the performance, durability, and efficiency of a chain and sprocket system or other comparable systems.